Image sensors are widely used in digital still cameras, security cameras, medical, automobiles, and other applications. Image sensors may work on the basis of pixel circuits. The pixel circuit usually comprises a photo diode and a plurality of transistors. The photo diode collects charges during exposition to light. The plurality of transistors transfers the charges to a bank of capacitors in which the charges are stored as voltages across the capacitors. The voltages are then read by suitable readout circuitry and processed to generate an output image.
Transferring of the charges to the bank of capacitors can be performed in various ways.
For example, a transfer switch electrically coupled to the cathode of the photo diode may be used to transfer the charges to a capacitive conversion node in the pixel circuit. The charges are stored in the capacitive conversion node. A pixel voltage may be sampled by a sampling circuit and referred to the bank of capacitors for subsequent readout.
In order to properly transfer the charges between the photo diode and the capacitive conversion node after exposure, the photo diode needs to be depleted of charges before exposure. This can be accomplished by resetting the photo diode so that the photo diode is depleted of charges before exposure. A common way to reset the photo diode is to electrically couple the cathode of the photo diode to a reference voltage via a reset switch. The reference voltage contributes to a reverse biasing of the photo diode. Depletion of the photo diode is enhanced with increased reference voltage. However, resetting the photo diode and capacitance, contributes to increased reset noise. Correlated double sampling (CDS) can be used to virtually eliminate the reset noise. After exposure of the photo diode, the capacitive conversion node is reset to its initial value, while the signal voltage is still stored in the photo diode. A reference voltage value is readout after exposure. Immediately after the reference voltage value is read out, the charges stored in the photo diode are transferred via the switch to the capacitive conversion node. After transferring the charges, a corresponding signal voltage value is readout. Thus the two readout values are said to be correlated. A difference between the two readout values represents the actual light induced voltage change. Since the two readout values are correlated, the reset noise, i.e. the noise contribution given by the reset operation substantially disappears.
In the example of the pixel circuit described above, the capacitive conversion node has typically a limited storage capacity which limits the use of the pixel circuit to a light intensity corresponding to the limited storage capacity of the capacitive conversion node. Higher light intensity will result in saturation of the capacitive conversion node and reduced dynamic range.
A number of solutions have been sought to obtain an extended dynamic range in a pixel circuit. For example, US patent application US2006/0071147A1 discloses a pixel circuit that operates in two different modes based on the light intensity level impinging on the photo diode. Under high light intensities, the capacitive conversion node is increased by turning-on the transfer switch during the readout operation. Alternatively, under low light intensities, the pixel circuit may operate with the transfer gate turned-off. However, the light intensity level is monitored before operating the pixel circuit in the two different modes. Further, since sampling occurs at different light conditions, systematic errors can be introduced in the sampling. Eventually, the systematic errors need to be calibrated out in a signal post-processing stage. In order to operate the pixel circuit disclosed in US2006/0071147A1, a cumbersome control of the pixel circuit is thereby required. There is a need for a simpler control of the pixel circuit.